One of the by-products created by the combustion of hydrocarbon (HC) fuels in burners that use atmospheric air is nitrogen oxide (NOx). A well-known problem in the industry for many years with the use of conventional burner designs utilizing preheated combustion air is higher flame temperatures which in turn contribute to an exponential increase in NOx emissions. Efforts to save fuel and increase combustion efficiency via recuperative and/or regenerative combustion systems combined with stricter governmental permitting laws for acceptable NOx emissions from furnaces has led to a much greater awareness and need to solve this problem in recent years. Ideally, the only by-products of stoichiometric hydrocarbon combustion should be water (H2O), carbon dioxide (CO2), and nitrogen (N2) with no carbon monoxide (CO), unburned hydrocarbons HC's, or NOx emissions. In addition, for maximum fuel efficiency the combustion reaction should proceed with as little excess air, which is air that is above the stoichiometric quantity of air necessary for complete combustion, as possible while minimizing the production of NOx. Another consideration is during cold furnace startup, when HC and CO emissions can be relatively high due to flame quenching from low furnace temperatures.
Techniques for controlling and inhibiting NOx formation in furnace combustion processes are well known and may include, for example, provisions for staging fuel, staging combustion air, recirculating flue gas into the burner, recirculating flue gas into the burner flame, altering combustion patterns with different degrees of swirl, and injection of water or steam into the burner or flame. Factors that contribute to the formation of NOx in burner-fired combustion chambers are the oxygen content of the flame or combustion chamber, the temperature of the combustion chamber, the temperature of the combustion air, the burner-firing rate, turbulence, and the residence time for complete combustion. It is known that NOx emissions increase with combustion chamber temperatures, the temperature of the combustion air, residence time, and typically with oxygen content in the combustion chamber. However, these factors are difficult to predict because burners for different industrial processes must operate at various furnace chamber temperatures, have various oxygen concentrations in the work chambers, may or may not have preheated combustion air, and are required to operate at different heat inputs depending on changing heat load requirements.
High Momentum (“HM”) burners, such as the burner disclosed in U.S. Pat. No. 4,431,403 and the burner disclosed in U.S. Pat. No. 4,443,182, both assigned to the assignee of the present invention are examples of burners that produce high levels of NOx during operation.
Previous efforts to solve the problem include the Staged Air, Low NOx Burner with Internal Recuperative Flue Gas Recirculation, U.S. Pat. No. 5,413,477. This design utilizes a combination of air staging and flue gas recirculation (FGR) for NOx reduction. However, the added capital expense for piping and controlling the recirculated flue gases are substantial.
Another embodiment by Bloom Engineering Co., Inc. details an air staged swirl burner, International Patent No. WO 01/35022 A1, for lower NOx emissions but does not address the cold furnace startup issue. In addition, there is still room for improvement in reducing the burner NOx emissions.
Finally, Tokyo Gas Co., Ltd. describes an air staging method for lower NOx emissions from burners incorporating regenerative beds in the burner body, U.S. Pat. No. 5,571,006. However, this design requires a separate ambient air connection to the burner body for flame stabilization and complete fuel burnout during cold furnace startups and/or below the auto ignition temperature of the fuel gas, again adding maintenance, installation, and operation costs.
What is needed is a gas burner that is capable of very low NOx emissions when fired on either ambient or preheated combustion air. The burner should also reduce carbon monoxide CO and hydrocarbon HC emissions during cold furnace startups. It should reduce emissions without the added expense of multiple air and/or fuel connections.